Click "ConversationList Settings" Then Choose "Contact Font",Click "Font Pack" drop down list, And you will see a list,for example: Handcent SMS font pack1,Handcent SMS font pack2,Handcent SMS font pack3,Handcent SMS font pack4,Handcent font pack5...choose one of these packs,if you haven't installed it ,handcent will navigate you to the market and you can install it directly. after you have choosen font pack ,you can choose font at the packs and define the style ,size and so on

You also can scan external font pack that you installed and apply them to Handcent SMS by going to "Custom Style" option and "Scan external Font packages"